Fix python docstring auto-fill (Bug#36056)

* lisp/progmodes/python.el (python-mode): Set
fill-indent-according-to-mode locally to t.  This lets auto-fill do
the right thing when auto-filling inside a docstring.  The default was
to nil on 2001-11-25 "(fill-indent-according-to-mode): Default to nil"
with the comment that it "screws up CC-mode's filling tricks".  But
presumably it shouldn't be a problem for python-mode.
* test/lisp/progmodes/python-tests.el (python-auto-fill-docstring):
New test.

lisp/progmodes/python.el

@@ -5379,6 +5379,7 @@ REPORT-FN is Flymake's callback function."
   (set (make-local-variable 'paragraph-start) "\\s-*$")
   (set (make-local-variable 'fill-paragraph-function)
        #'python-fill-paragraph)
+  (set (make-local-variable 'fill-indent-according-to-mode) t) ; Bug#36056.
 
   (set (make-local-variable 'beginning-of-defun-function)
        #'python-nav-beginning-of-defun)

test/lisp/progmodes/python-tests.el

@@ -1340,6 +1340,24 @@ this is an arbitrarily
      (should (string= (buffer-substring-no-properties (point-min) (point-max))
                       expected)))))
 
+
+;;; Autofill
+
+(ert-deftest python-auto-fill-docstring ()
+  (python-tests-with-temp-buffer
+   "\
+def some_function(arg1,
+                  arg2):
+    \"\"\"
+    Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua."
+   (auto-fill-mode +1)
+   (goto-char (point-max))
+   (newline)
+   (search-backward "Lorem")
+   (let ((docindent (current-indentation)))
+     (forward-line 1)
+     (should (= docindent (current-indentation))))))
+
 
 ;;; Mark